Method of calibrating microphones



May 20, 1952 1 J, M. KENDALL 2,597,005

METHOD OF CALIBRATING MICROPHONES Original Filed June 4, 1943 7 Sheets-Sheet, 1

grvumwib b 'J. M, KENDALL May 20,- .1952 I J. KENDALL I 2,597,005

. METHOD OF CALIBRATING MICROPHONES Original Filed June 4, 1943 r '7 Sheets-Sheet 2 grwe/rfim J. M. KENDALL a 20,1952 M..KENI DAL L 2,597,005

mum-10D .OF ,CALIBRATING MICROPHONES Original Filed June 4, 1943' "7 Sheets-Sheet 4 J. M. KENDALL gmw'm y 1952 J. M. KENDALL 00 METHOD OF CALIBRATING MICROPHONES Original Filed June 4, 1943 '1 Sheets-Sheet 5 J. M. KENDALL May 20, 1952 J, M. KENDALL 2,597,005

' METHOD OF CALIBRATING MICROPHONES V ori inai Filed June 4, 194: k 7 Sheets-Sheet s V grwmn/tm J. M. KENDALL May 20, 1952 J, EN LL 2,597,005

METHOD OF CALIBRATING MICROPHONES Original Filed June 4, 1943 7 Sheets-Sheet 7 grwmvbo'v J. M. KENDALL Patented May 2 0,

METHOD OF CALIBRATING MICROPHONES James M. Kendall, Coral Hills, Md., assignor to Geophysical Research Corporation, New York, N. Y., a corporation of New Jersey Original application June 4, 1943, Serial No. 489,669. Divided and this application July 1, 1944, Serial No. 543,180

(Granted under the act of March 3, 1883, as

7 Claims.

of application, Serial No. 435,614, filed March 21,

1942, now abandoned, for Seismic Surveying Equipment.

Many areas in which seismic surveys are conducted are covered with water. In surveying such areas, it is customary to mount the recording apparatus on boats. The seismic detectors usually used are microphones of a special type which are adapted for low frequency operation and are known in the art as geophones. The geophones may be of the same type as are used in land surveys but they are sealed'up water-tight and lowered to the bottom of the water body. If the bottom is of firm, solid material, this procedure usually works quite well. However, in the case where the bottom material is soft muck or ooze, a geophone resting thereon does not always give satisfactory results. In such cases, the practice usually has been to push the geophone down through the soft material until it rests on firm material. This is an awkward and difiicult procedure in shallow water and becomes practically impossible when the water is very deep.

In one form of the invention, a geophone or wave detector of any suitable type such as used on land is enclosed in a water-tight casing. The dimensions of the casing are so proportioned that the effective density of the assembly is slightly greater than the density of water. The geophone or wave detector is mounted at or close to one end of the container so as to make the center I of gravity of the device close to that end. As

the device almost but not quite floats, it will rest on the bottom but will exert practically no pressure thereon. Therefore, it will respond to any wave motion in the water surrounding it because its density is practically the same as that of the water. The wave motion in the water will move the detector just as it would move the water displaced by the detector, if the detector were not there, and the wave field will not be substantially distorted by the presence of the detector. This is true provided the wave length of the wave motion is large as compared amended April 30, 1928; 370 O. G. 757) with the dimensions of the casing. The velocity of seismic waves in water is approximately 5000 feet per second so that the wave length of a seismic wave of frequency equal to 200 cycles per second is 25 feet. The greatest linear dimension of the casing can usually be made of the order of one foot. Since the frequencies recorded in seismic prospecting are always less than 200 cycles per second, the response of the wave detector will be independent of wave length. As a matter of fact, by suitable design, the device can be made small enough so that frequencies up to at least 2000 cycles may be recorded.

For higher sound frequencies up to 10,000 cycles per second, the wave length of the sound begins to approach the dimensions of the microphone and the microphone 'no longer partakes of the particle motion of the water in responding to the sounds. It is then necessary to reduce the size of the microphone considerably but it has been found that when the microphone is made relatively small, bubbles which collect thereon produce inaccuracies in the microphone response by reason of the fact that the bubbles are compressible and each bubble covers a proportionately larger percentage of the microphone surface area that is true in the case where a large microphone is employed. It is, therefore, necessary to shape the microphone so that it can be easily debubbled.

The small microphone should also have a shape which easily lends itself to mathematical analysis for the purpose of computing the corrections which are necessary when the microphone is subject to sounds in the higher frequency ranges having a wave length approaching the dimensions of the microphone. It is also necessary that the shape of the microphone be such that it is fairly easy to compute corrections when the mass of the moving parts of the microphone differs from the mass of the water which the microphone displaces and thereby produces distortion in thewave field. The shape of the microphone case should be such as to provide as rigid a structure as possible thereby to eliminate as far as practicable resonance effects within the frequency rangein which sound measurements are to be made.

A second form of the invention has been devised to conform to the above-mentioned prerequisites. The second form comprises a microphone which has a substantially spherical casing comprised of two hemispheres having a water tight joint therebetween and is, therefore, capable of being easily debubbled because it presents no recesses within which bubbles may adhere. Since a sphere is symmetrical about all its axes, it lends itself to relatively simple mathematical analysis for the purpose of making the aforementioned corrections. A sphere is the most rigid structural shape obtainable and a microphone casing having a spherical shape will therefore have the least tendency to resonate at frequencies within the range to be measured.

One hemisphere of the sphericalmicrophone casing has rigidly mounted thereon and extending into the interior of the casing a coil which is associated with a magnetic structure mounted within the casing and flexibly'supported by the hemisphere upon which the coil is mounted thus providing a magnetic field in which the coil moves. The coil is provided with terminal leads which pass through packing glands in the microphone casing for connection to indicating apparatus located exteriorly thereof. The weight of the casing and the elements rigidly mounted thereon is made substantially equal to the weight of "the volume of Water displaced thereby when the microphone is suspended in a body of water. The weight of the magnetic structure which is flexibly supported within the casing increases the overall weight of the complete microphone structure considerably over the weight of the volume of water displaced but this produces no effect on the ability of the casing to partake of the particle motion of the water without substantial distortion of the wave field thereof. By reason of the flexible mounting of the magnetic structure within the casing, the inertia of the magnetic structure causes it to remain stationary as the casing moves in response to underwater sounds and the motion of the coil in the stationary magnetic field generates a voltage proportional to the velocity of the motion of the sphere and, hence, proportional to the particle velocity of the fluid.

To permit the casing to move freely within the body of water, a flexible suspension is provided therefor comprising a circular brass ring having'hooks placed at intervalsin its circumference. Thesplierical casing is provided with similarly spaced hooks and'flexible bands are arranged to extend from the hooks on the ring to the respective hooks on the casing. The ring may be supported from the surface of the body of water in any suitable manner.

It is sometimes desirable to measure the acoustical impedance-of the bed of a body of water. To secure such measurements, the velocity microphone is placed on the bed of the body of water with the axis of the coil normal to the bed and the response of the microphone to sounds is measured simultaneously with the acoustic sound pressure at substantially the same location. From these measurements, the acoustical impedance of the bed at that point may readily be determined.

The construction of the microphone permits determination of the absolute sensitivity thereof by the use of a simple calibrating device and method. To determine the absolute sensitivity of the microphone, the two hemispheres are separated and the hemisphere on which the coil and magnetic structure are mounted is affixed to a calibrating device which permits measurement of the change in flux linkages of the coil when the magnetic structure is moved a known amount with respect to the coil. The change in flux linkages is measured by a fluxmeter and the movement of the magnetic structure may be measured by either a microscope or a micrometer, a method being employed which insures accurate measurement thereby. A simple formula permits the translation of the two measurements into the absolute sensitivity of the microphone stated in microvolts/dyne/cmF.

As pointed out hereinbefore, a velocity microphone when constructed in accordance with the invention, does not substantially disturb the wave or sound field into which it is introduced and a measurement of the true field pressure is therefore obtained except at the high frequencies at which the wave length is comparable to the microphone diameter. This makes it possible to obtain the field response of a pressure microphone by placing it at a point at which the free field produced by an underwater loudspeaker has been calibrated by the velocity microphone, and the velocity microphone therefore may be utilized as a standard for the calibration of pressure microphones which are employed for the study of underwater sound pressures. The method of and apparatus for calibrating pressure microphones by employing the velocity microphone as a standard will be more fully described hereinafter.

The term pressure as employed herein refers to the acoustical sound pressures within a body of water and is in no way related to the hydrostatic pressure occurring in the water by reason of the hydrostatic head thereof.

One of the objects of the present invention is the provision of a new and improved underwater wave detector or microphone possessing allof the advantages of devices heretofore proposed for this purpose andin which the foregoing disadvantages have been eliminated.

Another object of the invention is the provi-- sion of an underwater microphone which responds to the particle motion of the water without substantially distorting the wave field thereof.

Still another ob ect of the invention is the provision of a new and improved microphone which will aid in the accurate determination of the particle velocity of the bed of a body of water in the presence of sound waves for the purpose of determining-the acoustical impedance of said bed.

An additional object ofthe present invention is to, provide an-underwater microphone which is adapted to be easily debubbled and has a simp egeometrical surface which readily lends itself to mathematical analysis.

A further object of the invention residesin a novel and improved arrangement whereby-underwater sounds having frequenciesup to and beyond 10,000 cycles persecond may be easily measured.

Still another object is the provision of an underwater velocity microphone which is sufficiently accurate in its response to serve as a primary standard for the calibration of other underwater microphones. v

Another of the objects of the present invention resides in the provision of a novel method of and apparatus for determining theabsolute sensitivity of a velocity microphone.

Still another object of the invention'is to provide a novel method of and apparatus for calibrating an unknown microphone by employing a velocity microphone as a primary standard.

Still other objects, advantages and improvements will become apparent from the following detailed description taken in connection with the accompanying drawings in which:

Fig. 1 is a diagrammatic view partly in section of one form of the apparatus in accordance with the invention;

Fig. 2 is a view in elevation of a preferred form of the invention and illustrating a spherical velocity microphone and the manner of supporting same;

Fig. 3 is a view in section of the microphone of Fig. 2 taken substantially on the line 3-3 thereof;

Fig. 4 is a view taken on the line 4-4 of Fig. 3 and illustrating the arrangement for centering the magnetic structure of the microphone with respect to the coil carrying element;

Fig. 5 is a perspective view of an apparatus suitable for determining the absolute sensitivity of a velocity microphone;

Fig. 6 is an enlarged view partly in section and partly broken away of a portion of the apparatus of Fig. 5;

Fig. '7 is a diagrammatic view of the apparatus employed for calibrating underwater microphones;

Fig. 8 is an enlarged view of a portion of Fig. '7 and illustrating the manner in which certain of the apparatus is supported;

Fig. 9 is a view taken on the line 2-9 of Fig. 8;

Fig. 10 is a. diagrammatic view of the electrical apparatus employed for the purposes of calibration and utilizing a velocity microphone as a standard; and

Fig. 11 is a view illustrating the apparatus of Fig. 10 with the velocity microphone replaced by an unknown microphone.

Referring now to the drawings in which like numerals of reference are employed to designate like parts throughout the several views, there is shown in Fig. 1 a complete system according to one embodiment of th invention. As indicated in this figure, the seismic operations are conducted from a boat It in which are provided the necessary recording and control equipment which is not shown in detail. Another boat, not shown, is equipped with devices for lowering and firing the explosive charge by which the seismic waves are generated.

The wave detector or geophone I I is located at one end of a cylindrical casing or tank I2, the size of which is dependent upon the weighted size of the geophone. The size and material of the casing are such that the effective density of the assembly is just slightly greater than that of the water covering th areas to be surveyed so that the assembly rests on the bottom without exerting any appreciable pressure thereon. A twoconductor insulated cable i3 leads from the geophone through the side of the casing near its lower end to the recording equipment it contained in the boat. This cable serves to conduct electrical impulses from the geophone to the recording equipment and may also be used for lowering the geophone to the bottom and raising it therefrom, or a separate cable may be provided for this purpose.

In the use of this device for seismic surveying, it is lowered to the bottom and because of the arrangement of the geophone at one end, such end contacts the surface of the bottom without exerting appreciable pressure thereon. Th assembly will respond to any wave motion in the water without substantial distortion of the wave field. Seismic waves are generated in the usual manner and the geophone detects th waves which pass up through the earth into the water and produce wave motion therein corresponding to the wave motionproduced in the earth. Elec trical impulses corresponding to the wave motion are impressed by the geophone on the recording equipment in th usual manner. The condition of the bottom has no effect on the response of the geophone since it is fully responsive to the wave motion in the water.

Referring now to Figs. 2 to 11 inclusive and more specifically to Figs. 2, 3 and 4, a preferred embodiment of a device for measuring underwater sounds is illustrated and comprises a microphone designated generally by the numeral l5, the microphone having a. substantially spherical shell I6 about 2 inches in diameter which is made of Duralumin or any other suitable light weight metal. The shell is divided into two substantially hemispherical sections I1 and I8 connected to each other as by threads on the male portion 19 of the section I1 and the female portion 2! of the section l-8 to form a Water-tight joint therebetween at the machined abutting faces 22 of the two hemispherical sections.

Screwed into the section I? is a thin cylindrical coil form 23 made of plastic or any other suitable material about five-eighths of an inch in diameter on which is wound a coil 24 which, by way of example, may comprise about 250 turns of #39 Formex magnet wire. The coil 24 is provided with terminal leads 25 and 26 supported within the section ll as by an adhesive 21, the leads 25 and 28 being joined to insulated conductors 28 and 29 respectively which pass through watertight packing glands 3i and 32, respectively, arranged in th heinipsherical section IT. The packing glands 3| and 32 may be of any conventional construction but the metal portions thereof should preferably be made of a light weight metal such, for example, as Duralumin.

Flexibly supported within the hemispherical section i! as by the soft rubber blocks 33 is a magnetic structure designated generally by the numeral 34 and comprising a cylindrical soft iron pole member 35 having a. circular aperture 36 therein, the coil 24 being located within the aperture 36 which is slightly greater in diameter than the coil so as to provide a small amount of clearance therefor. The rubber blocks 33 are afixed to the pole member 35 by a suitable adhesive and support the pole member 35 centrally of a cylindrical seat 31 within the hemispherical section ll'.

Th cylindrical pole member 35 has attached thereto as by screws 38, a circular soft iron plate 39 having a circular aperture 4| therein within which is Welded as indicated at 32 a cylindrical extension 43 of a permanent magnet 44. The magnet 44, which is preferably made of Alnico, is square in cross-section and has the end 45 reduced to a circular form to provide a surface to which a cylindrical soft iron pole piec 46 is welded as at 47.

The pole piece 46 has a diameter slightly less than the internal diameter of the cylindrical coil form 23 so that it is freely movable within the coil form in juxtaposition to the coil 24. To properly center the pol piece 46 with respect to the coil form 23 and yet permit free movement therebetween, a flexible rubber disc 48 having four arms 49, as shown in Fig. 4, is clamped to the top of the pole piece 456 as by a screw 51 and a washer 52, the arms 49 lightly gripping the interior of the coil form 23 and thereby flexibly centering the pole piece 4'6 with respect thereto.

For supporting the microphone [5 in a body of water so as to be substantially free of mechani- 7 cal restraint, the shell 16 has embedded'the'rein' a plurality of eyes 53 arranged-in the same plane and spacedsymmetrically about the circumference of theshell. A brass ring 54 about'10 inches in diameteris provided with symmetrically spaced eyes 55 and flexible supporting means such as rubber bands 56 are arranged between the respective eyes' 53 and 55, the shell It being substantially free to move in a direction normal to the plane of the ring 5 3. The conductors and 29" are clipped to the ring 54 by clips 51, the ring being provided with an upstanding member 58 through-which the conductors 28 and '29'pass to formthe cable 59 held to the member 53'as by a clainping nut 61. The conductors 28 and 29 are provided with-suflicient slack between the-shell lfi a-nd the-ring 54 thereby to permit the shell to have substantial freedom of movement.

--The-spherical shell IG and all the parts rigidly afiixedthereto, suchas the coil form 23, the coil 24, the leads 25 and-26 and the packing glands 3| and, are so designed that the weight thereof will substantially equal the weight of the volume ofwater displaced thereby when the shell is submerged in water. For all underwater sound frequencies Well above the resonant frequency determined by the mass of the magnetic structure and the flexible support therefor, the inertia of the magnetic structure 25 prevents its moving appreciably. The shell l6, being substantially free of mechanical restraint by reason of its flexible connection to the magnetic structure and to the supporting ring 54, will have a motion substantially identical with the fluid particle motion and will not distort the wave field produced by the underwater sounds. The motion of the 001124 in-the stationary magnetic field which extends between the pole member 35 and the pole piece 46 generates a voltage proportional to the velocity of motion of the shell l6 and which is, therefore, proportional to the particle velocity of the fluid.

The spherical shape of the shell 16 possesses several advantages. As hereinbefore pointed out air bubbles which collect on the surface of a submerged microphone are a source of inaccuracy because of their compressibility thereby preventing the sphere from truly responding to the fluid particle motion. A sphere is easily debubbled because of the absence of recesses therein Within which bubbles may collect. Such bubbles which may collect thereon when the microphone I5 is submerged may be removed therefrom by wiping with a wet cloth or by forcing a stream of water thereagainst and, since the bubbles are easily visible, it is a simple matter to discern whether theshell l5'of the microphone is'entirely free of bubbles.

Another advantage possessed by the spherical shell is that its simple symmetrical geometrical shape lends itself to accurate mathematical calculation for any departure from the true fluid particle motion which may be caused either by any small difference which may exist between the weight of'the shell l6 and the weight of the volume of water displaced thereby or by the fact that the wave length of the sound being measured is relatively short and therefore begins to approach the diameter of the microphone shell. The manner in which such calculations are made will be apparent to those skilled in the art and forms no part of this invention.

It is necessary to determine the absolute sensitivity-before any underwater measurements-made by the microphone can be given their proper intrpretation. The manner in which theabsolute 8 sensitivity'*of the -microphone can be determined will'now be'descr-ibed in-connection 'with thecalibration apparatus required for this purpose and illustrated in'Figs; 5 and 6.

The calibrationapparatus comprises a fixture shown generally by the numeral62 and a microscope shown generally by the numeral 63. -The fixture 62 has-a base t l supporting at 'one'end thereof as by screws 65, an upstanding plate 66 having therein a threaded circular openingfil adapted to receive the male portion 4 9' of the shell IG. -Also mounted on the base 64 and fastened thereto in any suitablemanner is an upstanding plate-68 havinga-pair of clamps fill adjustably fastened to the upper edge thereof as by screws ll each adapted to be screwed into any one of a plurality of threaded holes 12. The clamps 69 are adapted to adjustably fasten a micrometer 13 to the upper edge of plate 68.

The base 64 also has fastened thereto asby machine screws 15 and nuts 10 a flat plate 16 andan anglemember H, the latter having a circular aperture therethrough for receiving a freely slidable shaft 18. The shaft 18 has threadedly connected to one end thereof a screw element 19 which is adjustably held in any desired position with respect to the shaft 18 by a locknut 85. The opposite end of the screw element It has rigidly fastened thereto by-the pair of lock-nuts 82 a coupling bar83 which is adapted to be fastened to the plate 39 of the'magnetic structure 34 of the microphone by several of the screws 38 which have been previously removed for this purpose. The rod 18 also has mounted thereon a collar 84 which is adjustable'along the shaft and may be clamped in any position thereon 'by the knurled set-screw 85, the collar 84 being placed between the'measuring faces of the micrometer 13. A removable spring 86 is arranged to bias the collar 84 away fromthe' member Tl for a purpose which 'w illbecome more apparent as the description proceeds.

The microscope 63 is provided with a' base'8'l upon which the fixture 62 is clamped by any conventional means in a position such that the objective lens 88 of the microscope is centered-on the shaft 18 regardless of the adjustment of the shaft. The microscope is-also-provided with a filar micrometer eyepiece having a cross-hair adjustment dial 851* which, when properly calibrated is employed to make very exact measurements in a well known manner.

The manner in which the calibration apparatus is employed to determine the absolutes'ensitivity of the microphone 15 will now be described. The hemisphere I8 of the microphone is removed and the male portion I9 of the hemisphere I! is screwed into the opening 61 of the plate 66. Two diametrically opposite screws 38 are removed from the magnetic structure 34 and the coupling bar '83 is attached thereto by means of the samescrews '38. The spring '86 is removed and the set-screw loosened to permit the collar 84 to slide freely on the'shaft 18 and permit the magnetic structure '34 to assume its normal position within the hemisphere H as determined by the unstressed condition of the rubber blocks 33 which support the magnetic structure within the hemisphere. The measuring faces of the micrometer 13 are separated by a distance equal to the width of the collar 84 plus 0.015 inch, the micrometer faces being locked in this position, and the collar 84 is clamped to the shaft 'IB -by'the set-screw'85substantially centrally of the distance between themicrometer 9 measuring faces or about 0.0075 inch from either face. This distance is not critical as the exact measurements will be made by the microscope 63.

The spring 86 is again placed between the member 11 and the collar 84 thus biasing the collar against the movable measuring face of the micrometer and simultaneously moving the magnetic structure 34 approximately 0.0075 inch from its normal position. A fluxmeter (not shown) is connected to the conductors 28 and 29 for measuring the flux linkages produced when the magnetic structure 34 is moved 0.015 inch with respect to the coil 24. Aluminum dust is sprinkled on the shaft I8 directly below the objective lens 88 of the microscope and the crosshair of the microscope is centered on one side of a selected particle of aluminum dust. A reading of this position is taken on the microscope scale and cross-hair adjustment dial 89.

The collar 84 is manually moved against the bias of the spring 86 until it strikes the stationary measuring face of the micrometer and thus causes a displacement of approximately 0.015 inch of the magnetic structure 34 to the other side of the normal position thereof with respect to the coil 24. The cross-hair of the microscope 63 is again centered on the same side of the same particle of aluminum dust on the shaft 13 and another reading on the scale and the cross-hair adjustment dial 89 is taken. The fluxmeter reading is also taken and the foregoing procedure is repeated a number of times to secure a good average result.

The absolute sensitivity of the microphone for plane waves may now be determined from the following formula:

where S=absolute sensitivity in microvolts/dyne/cmF.

K =fluxmeter sensitivity in linkages/ division.

A=fluxmeter deflections in divisions.

B=displacement in microscope divisions.

D=cm./microscope division.

R=specific acoustic resistance of water.

=1.43'l 10 dyne sec./cm.

If a microscope is not available, fairly accurate results may be obtained by employing the readings of the micrometer I3. When these readings are substituted for those of the microscope, two of the factors of the formula above must be redefined as follows:

B=displacement of micrometer in inches. D=2.54 cm./inch.

It will be noted that the above formula applies only when determining the absolute sensitivity of the microphone for plane waves. However, the microphone must be operated at a relatively small distance from the sound source in order that pressures under investigation will be appreciably greater than those of background noises and reflections. Hence, the sound waves at this distance will be spherical rather than plane waves. The relation of particle velocity for plane waves to that for spherical waves, at points where the pressures are equal, is

where o =particle velocity for plane waves. vs=particle velocity for spherical waves. r=distance between sound source and detector. c=velocity of sound in water.

f=frequency in cycles per second.

Referring now to Figs. '7 to 11 inclusive, an apparatus and method is disclosed for calibrating a pressure responsive condensentype microphone employing the microphone I5 as a primary standard. I

The microphone calibrations are conducted in a tank Si or other body of water which is substantially free from currents, waves and noise. The water should be at least fourteen feet deep and the tank must be at least equally wide and long to prevent serious reflection of sounds from the surface of the water or from the bottom and side walls of the tank.

The velocity microphone I5 to be employed as a standard is suspended by adjustable links 92 fastened at one end to the brass ring 54 and at the other end to a wooden support 93 by any suitable means. Similarly mounted by means of adjustable links 94 about one foot away from the microphone I5 is an underwater loudspeaker 95 having a supply cable 96 connected thereto,

the microphone I5 and the loudspeaker 95 being so placed with respect to each other that the axis of the microphone coil 24 is in alinement with the axis of the loud speaker diaphragm.

The wooden support 93 is suspended at the proper depth within the tank 9| as by a pair of links 91 fastened at one end thereof to the support 93 by any suitable means and joined together at the other end by a fixture 98 fastened to a raising and lowering cable 99, the two ends of which pass over pulleys IOI fastened to the ceiling I02. The ends of the cable 99 are adapted to be wound around individual wall cleat I03 which permit adjustment of the microphone and loudspeaker at any desired position within the tank 9|. The microphone cable 59 and the loudspeaker cable 96 are connected to the calibrating apparatus, shown generally by the numeral I04, which rests upon a table I05.

The calibration apparatus IDA is more fully illustrated in Figs. 10 and 11 and comprises an oscillator I05 which is adjustable by means of a control knob I0! through a frequency range of 100 to 10,000 cycles per second, this being the range of sound frequencies through which it is desirable to calibrate a pressure microphone. The output of the oscillator IE6 is adapted to be connected by the transfer switch I08 either across a resistor I09 in series with the microphone I5 or to an amplifier III having a volume control H2. An amrneter H3 is provided to indicate the value of the current being supplied to the resistor H39 by the oscillator I06 which is provided with a volume control H0 for varying this current. The output of the amplifier III is connected to the loudspeaker 95 through the cable 95, a voltmeter H4 being connected thereacross.

The cable 59 of the microphone I 5 is connected to a preamplifier I I5, the output of which is connected to an amplifier I I6 having a volume control II: and thence to a rectifier H8. The output of the rectifier H8 is connected to a recordin instrument II9 provided with a pen I2I and a cylindrical drum on which is carried a recording chart I22. The cylindrical drum is provided with a central shaft I23 having a pulley (not 11' shown) fixed on one end which is driven 'by-a belt I24 operated from a' pulley (not shown) which is fixed'to a shaft I25 geared to'the frequency control knob Iii? of the oscillator I06.

The apparatus so far describedis employed for the purpose of calibrating the output of the loudspeaker at a point in the water one foot away; that is, atthe point at which the standard velocity microphone I is located. Having once done sojit'isafi simple matter to calibrate an unknown pressure microphone by replacing the standard microphone I5 by the microphone to be calibrated and recording its response to the loudspeaker 95 underia similar set of conditions.

Referring nowto'Fig. 11, the apparatus of Fig. Iil' isillustrated as it appears after the unknown microphone'shown generally by the numeral I26 has been substituted for the standardmicrophone I5, the microphone I26 being supported by the Wooden support 93 (Fig. 8) by links in a similar fashion to that employed in supporting the microphone I5 and in exactly the same position with respect" to the loudspeaker 95. The unknown microphone in the present example comprises a condenser transducer I 21 connected in series with a resistor I2 8'to theconventional built-in preamplifier I2 9', the output of which is connected by the cable I3 I to the amplifier II 6 in a mannersimilar to the arrangement of Fig. 10. The resistor' l 28 is adapted to be connected by the cable IS'I to'theosoillator' IBS-in the upper position of'the switch I08.

The me'tliodwhereby the pressure microphone I26 .is calibrated withrespect to-the standard microphone I5 will nowbe described. Let it be assumed that fthe' standard microphone I5 and the loudspeaker 95 have been submerged to a proper depth in thetank 91 as illustrated in Fig. 'Zfa'nd 'the'apparatus I 04 has beenconnected as illustrated-in Fig. 10. The pen I2] is raised from thechart I22 and the'knob IU'I'is adjusted to set theoscillator I06 at its lowest frequency generatin position of 100 cycles per'second, the oscillatormeanwhilebeing' permitted to remain unener'giz'ed'. The'switch' I08 is placed "in' its upper position thereby disconnecting the loudspeaker 95from the oscillator and connecting the oscillator across the resistor I89 in circuit with the microphone I5; The oscillator IDS-is nowenergized and its volume control IIIl isadjusted until the' ammeter IIS'readS'a current value which will produce for purposes of illustration about 50' microvolts across the resistor I09, this current value remaining fi'xed during the subsequent measurements. The pen I2I'is-* now replaced on the chart I22, the volume control III having previously been adjusted to a value which experience has taught will give a satisfactory trace on the chart.

The frequency control knob IIl'I'is' now slowly rotated through the entire range-of the oscillator I06 from 100 to 10,000 cycles per second, the belt I24 slowly operating the chart I22 in synchronism with the control knob I01. Simultaneously, the pen I2l will trace on the chart I22 the response ofthe elements II 5, H6, H8 and ll9for each frequency supplied thereto by the oscillator through its connection to the resistor I09 thereby calibrating theseelements at the various frequencies, it being understood that the lcharti' I22 has properly marked thereon indicia representative'of the frequencies supplied by the oscillator I06;

The switch Wis now moved to its lower'position thereby disconnecting the oscillator from resistor IIiB -andconnecting it to theloudspeaker and the volume control I [2 is adj usted-until-a satisfactory input as indicatedby the voltmeter H4 is supplied to 1the-= loudspeaker. The frequency control'knob III'I' is again rotated-whereby asecondi'traceis placed on the chart' l22'; the latter trace being representative of the response at'each frequency "of 5 the microphone .:I 5;

F'rointhetwo traceson the chart-I22 and "the absolute-sensitivity of the microphone I5' deter mined with the apparatus-of Figs. 5'and 6 as hereinbefore described; the calibration of the loudspeaker output in dynes/cm. at a point one footaway for any or all" frequencies-may easily be determined; For any"particularfrequency; the calibration of the loudspeaker output in dynes/cm equals -the ratio 'ofthe magnitude of thesecond trace -at that frequencyto the magnitude of the first trace at the same frequency multiplied by the number of microvolts placed across the resistor I09; the resultant being divided by the absolute sensitivity of themicrophone I 5 in microvolt's/dyne/cm To calibrate anunknown pressure microphone I26- for the range of frequencies from to 10,000cycles persecond, the support 93 (Fig.- 7) is raised from the'tank-SI by the cable 99 and the unknown pressuremicrophone is substituted for the standard microphone I5" in'the manner shown diagrammaticallyin Fig. 11. The pres sure microphone and loudspeaker are submerged in the tank, it being understood-"that-the pressure microphone I26 is placed in exactly thesame spacerelation to the-loudspeaker 95-as the standard microphone bore to'it." The volume controls III], 2 and Ill are allowed toremain in'eXa'ctly the same positions to which they are adjusted in making the calibration: of the loudspeaker output.

The switch I08 (Fig. 11) is placed in its upper position and a first trace is made on the: same record chart. I22 preferably. in a different colored ink than: was employed makingthe earlier traces, the traceibein'gimadefbgrotating the frequency control knob I Ill as heretofore. The-reading of the ammeter I I3i'snoted as it will differ considerably from the readingnotedin'Fi'g; 10, the value of the resistor I28 beingconsiderably higher than that of theresistor'l (III-by reason of the much greater sensitivity. of the condenser transducer IZT over'that of the velocity microphone I5. By"multiplyingl the reading of the ammeter I I3"by the resistanceofthe'resistor' I 28, the microvoltsapplied thereacross may be determined. The switch l08is now'moved to its lower position and a trace in still another colored ink is made upon the chart I22by rotatingth frequency control knob I 0T. 7

From the latter two traces taken in connection with the calibrated loudspeaker 'outputderived from-the'first two traces; thecalibrationsof the" pressure microphone I26 atany frequency may readily be-determined'. For any" particular frequency, thercalibration of the pressure microphone is microvolts/dyne/cmz equals the ratio of the magnitude: ofthe second-pressure microphone trace at-that frequency to the magnitude of-the first'pressure microphone trace at the same frequency multipliedby the number ofmicrovolts applied acrossresistor I28, the resultant being divided by theloudspeaker output calibrationat the same frequency in dynes/cm.

It will, of course, be understood that each of the hereinbefore mentioned corrections which may be applicable under a particular set of conditions is applied in determining the calibration of the pressure microphone I26 thereby to eliminate inaccuracies produced when the wave lengths of the sounds being measured approach the diameter of the microphone l5, or the weight of the shell I6 is diiferent from that of the volume of water displaced thereby, or the distance between the loudspeaker and the microphones during measurements is relatively small.

The velocity microphone of this invention may also be employed for determining the acoustical impedance of the bed of a body of water. The ring 54 is disconnected from the links 92 and the ring, with the microphone l still mounted therein, islaid horizontally upon the bed of the body of water at the point at which the acoustical impedance is to be determined. The ring 64 performs the function of maintaining the axis of the coil 24 normal to the bed of the body of water so as to place the coil in a position wherein it will respond to the particle motion at the interface between the water and the bed by reason of the fact that the microphone shell l6 has practically the same density as the water.

A pressure type microphone, such as the microphone IZS of Fig. 11, is supported adjacent the microphone l5 and both microphones are subjected to sounds of various frequencies, so that both the acoustical velocity and the acoustical pressure of the sounds may be simultaneously recorded on suitable instruments located at the surface of the water. The acoustical impedance of the bed of the body of water at any particular frequency can be determined from the formula where Z==acoustical impedance P =acoustical pressure V=acoustical velocity Briefly stated in summary, the present invention contemplates the provision of a new and improved underwater sound wave detector or microphone adapted to respond to the fluid particle motion of the water without substantial distortion of the wave field thereof. In the preferred embodiment, the microphone is so shaped that it may be readily debubbled and lends itself to mathematical analysis for the purpose of calculating the corrections which are necessary when there is either a difference in the mass of the microphone from the mass of the water which it displaces or when the wave lengths of the sounds to be detected approach the dimensions of the microphone. The invention further contemplates the provision of a novel method of and apparatus for determining the absolute sensitivity of the microphone of the present invention and also a new and useful method of and apparatus for employing the microphone of the instant invention as a standard for calibrating other microphones. I

Although, in accordance with the provisions of the patent statutes, this invention has been described in concrete form with reference to a preferred embodiment thereof which gives satisfactory results, it will be understood that this form is merely illustrative and that the invention is not limited thereto since alterations and modifications will readily suggest themselves to persons skilled in the art without departing from the true spirit of this invention or the scope of the annexed claims.

The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. The method of determining the absolute sensitivity of a velocity microphone which comprises: the first step of displacing the magnetic structure of the microphone to one side of its normal position with respect to the microphone coil, the second step of displacing the magnetic structure of the microphone to the other side of its normal position with respect to the microphone coil, measuring the distance traversed by the magnetic structure during said second step, and measuring the change in the flux linkages of the microphone coil during said second step.

2. The method of determining the absolute sensitivity of a velocity microphone having a magnetic structure and coil associated there with and movable relative to each other which comprises: securing a rigid control member to the movable element of the microphone, placing a small quantity of powder on said control member, moving the control member outwardly from the microphone, measuring the distance through which the control member has moved by inspecting the degree of movement of a selected particle of said powder, and measuring the flux linkages cut by the microphone coil during the measured movement of said movable element.

' 3. The method of determining the absolute sensitivity of a velocity microphone which comprises: the first step of displacing the magnetic structure of the microphone to 2. moved position at one side of its normal position with respect to the microphone coil in which the characteristic of flux linkages cut per unit distance of movement is constant, the second step of displacing the magnetic structure of the microphone from the first moved position to the other side of its normal position with respect to the microphone coil in which the characteristic of flux linkages cut per unit distance of movement is constant, measuring the distance traversed by the magnetic structure during said second step, and measuring the change in the flux linkages of the microphone coil during said second step.

4. The method of determining the absolute sensitivity of a velocity microphone which comprises: the first step of displacing the magnetic structure of the microphone to a moved position at one side of its normal position with respect to the microphone coil in which the characteristic of flux linkages cut per unit distance of movement is constant, the second step of displacing the magnetic structure of the microphone from the first moved position to the other side of its normal position with respect to the microphone coil in which the characteristic of flux linkages cut per unit distance of movement is constant, measuring the distance traversed by the magnetic structure during said second step, measuring the change in the flux linkages of the microphone coil during said second step, and calculating the absolute sensitivity of the microphone from the ratio of the measured change in said flux linkages to the distance measured.

5. The method of calibrating an underwater microphone of the pressure type by utilizing a loudspeaker and a microphone of the velocity type having a magnetic structure and a coil associated therewith, which comprises, displacinezrsald masn tic.-. tn1c ure.a easu st n with; respect t sa dv c ilim asur n th flu ink ages 1 cutby .said coil; during I such displacement, computing from the-displacement and fluxlinkages the response of said-velocity microphonein volts per unitof pressure appliedthereto when I the microphone is immersed; in water, submerging said-J velocity microphone and a loudspeaker a predetermined distance-apart inwater; calibrating, said loudspeaker with respect-to said; computations, replacing the velocity microphone by the. pressure microphone, and calibrating said pressure microphone with respect to said loudspeaker.

,6; The method ofcalibrating an underwater microphone of the pressure type by utilizing a loudspeaker and a microphone of; the velocity type having two elements comprising a magnetic structure and coil associated therewith, one: of said structure and coil being movablerelative to the other, which comprises, securing a rigid control member to the movable element of the velocity microphone, placing a small quantity of aluminum powder on said control member, moving-the control member outwardly from the velocitymicrophone, measuring the distance through which the control member has moved by inspecting: the degree of movement of a selected particle of; said aluminum powder, measuring the flux linkagescut by said coil during the'measured movement ofsaid movable element, computing from the displacement and flux linkages the response of said velocity microphonein volts output per-unit of applied pressure when the microphone isimmersed in water, submerging a loudspeaker and-saidivelocity microphonea predetermined distance, apart within a body of water with theaxis of the velocity microphone directed toward the sound radiating element of the loudspeaker, applying electrical signals of pre determined': frequencies and magnitudes to the loudspeaker, recording the-response, of the velocity microphone to each of said signals, replacing thevelocity microphone by the pressure microphone,- reapplying, electrical signals of saidpredetermined frequencies and magnitudes to the loudspeaker, recording the response of the pressurezmicrophoneto-each of said signals, and calibrating-thepressuremicrophone by utilizing said 16 omputation nd paring: the? e or ed, rersponses ofthe microphones to said signals.

7. The method of calibrating an underwater microphone of the pressuretype by, utilizing a loudspeaker and av microphone of theyelocity type having two elements comprising a. magnetic structureand'coil associated therewith one-ooi said structure and, coil being movable relative, to the other, which comprises, displacing the magnetic'structureto a moved position-at-oneside of its normal position with respect-to said coil in which thecharacteristic of flux linkages cut per unit distance of movement is constant, displacing, the magnetic structure from the first moved positionto the other side of itsnormal position with respectto the microphonecoil in which thechara acteristic of flux linkages cutper unitvdistance of move ment is constant, measuring the distance. traversed by-the magneticstructure duringthe last nameddisplacement, measuring the change in fiuxlinkages of the coil during the last named displacement, computing from; the last named displacement and flux linkages the responsev 01 said velocitymicrophone in volts output perunit of applied pressure when the microphone, is immersed in water, submerging a loudspeaker and, said velocity microphone a predetermined distance apart within a body of water, utilizinglsaid computation for calibrating ,said loudspeaker with respect to the velocity microphone, replacing the velocity microphone by the pressure microphone, and calibrating said pressure microphone with respect to said loudspeaker.

JAMES M. KENDALL,

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,134,316 Collette Apr, 6, 1915- 1,412,405 Herrmann Apr. 11, 1922 1,932,901 Harrison Nov. 24, 1931 2,231,085 Morrison et al Feb. 11, 1941 2,265,292 Krebs Dec. 9, 194:1 2,357,353 Pearce Sept. 5,1944 2,425,361 Brown Aug. 12,- 1947 

